The subject matter described herein relates generally to stress sensing in ferromagnetic materials, and more particularly, to non-contact sensors, systems, and methods for the sensing of stress in ferromagnetic materials.
Most known ferromagnetic materials have a magnetostrictive property that induces the materials to change shape in the presence of an applied magnetic field. The inverse is also true, i.e., when a force is induced on a ferromagnetic material, the magnetic properties, such as the magnetic permeability, of the material change. A magnetostrictive sensor senses the changes in magnetic permeability and, because the changes are proportional to the amount of stress applied to the ferromagnetic material, the resulting measurements are used to calculate the amount of stress.
The changes in the magnetic permeability due to stress applied to the ferromagnetic material, however, is small, making accurate measurement difficult. At least some known magnetostrictive sensors are used with a ferromagnetic material that has had a magnetic field permanently induced in at least a portion of the material to facilitate measuring the stress in the material. This feature requires magnetically encoding the material to be sensed. Such an operation is typically expensive and not all materials will retain their magnetization over long periods. In addition, such an operation makes it more difficult to retrofit existing systems with a magnetostrictive sensing system because the material to be sensed, e.g. a shaft of a gas turbine, has to be temporarily removed from the system to undergo permanent magnetic encoding. In at least some other known magnetostrictive sensing systems, a pre-conditioning magnetic flux device is used to induce a conditioning magnetic flux into the material to be sensed and a separate magnetic field detector senses transient changes to the magnetic field. At least some of these systems make it more difficult to retrofit existing apparatus or machines with a magnetostrictive sensing system because of the need for multiple devices. In addition, at least some known magnetostrictive sensors use special ferrite core designs and require use of specialized winding-machines during manufacturing of the sensors. This leads to increased manufacturing costs as well as increased size of the sensors.